The present invention relates to electrodes for use in secondary electrochemical cells. More particularly, it concerns a method of making a negative electrode composition, the electrode composition made thereby and the secondary electrochemical cell containing the electrode, wherein the negative electrode composition includes a lithium alloy including silicon and nickel.
A substantial amount of work has been done in the development of high temperature secondary electrochemical cells of the general type described in Shimotaki et al, U.S. Pat. No. 3,488,221 issued Jan. 6, 1970 and in particular in my prior U.S. Pat. No. 4,764,437 issued Aug. 16, 1988 assigned to the assignee of this invention, the entire disclosure of which is incorporated herein by reference. Typical secondary cells have long shelf lives, may be completely and repeatedly charged and discharged at either rapid or slow rates, and can produce extremely high currents for short periods of time. Secondary cells of this general type have uses in space and other remote areas.
In such high temperature secondary electrochemical cells discussed above, the positive electrode generally has been formed with chalcogens such as sulfur, oxygen, selenium or tellerium, as well as their transition metal chalcogenides, and have included iron disulfide in addition to the sulfides of iron, cobalt, nickel and copper.
In high temperature cells, current flow between electrodes often is transmitted by molten electrolytic salt. Particularly useful salts include compositions of alkali metal halides and/or alkaline earth metal halides ordinarily incorporating a salt of the negative electrode-active metal, such as lithium, see column 2 of the Shimotaki et al, U.S. Pat. No. 3,488,221. One problem with many of the electrolytes available is the limited dynamic range for which the electrolyte will remain liquid, at a specific temperature, to avoid the electrolyte from solidifying in the electrodes as the concentration of positive ions changes during cell operations.
Alkali metals such as lithium, sodium, potassium or alkaline metals including calcium, magnesium, etc. along with the alloys of these materials, have been used as negative electrode active materials. Alloys of these materials such as lithium-aluminum, lithium-silicon, lithium-aluminum-silicon, lithium-magnesium as well as many others have been used to improve retention of the electrode material at the high operating temperatures of these secondary electrochemical cells.
In the past, negative electrode materials were prepared by various metallurgical techniques such as those disclosed in U.S. Pat. No. 4,489,143 issued Dec. 18, 1984, assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. In that patent, the negative electrode was prepared by forming a molten alloy of iron and aluminum. The melt was solidified and comminuted to particles of aluminum-iron alloy in the specified proportions. Thereafter, the alloy particles were integrated into a porous, electrically conductive substrate. Other metallurgical techniques such as casting or extruding are also discussed.
A secondary electrochemical cell and a negative electrode is disclosed in U.S. Pat. No. 4,324,846 issued Apr. 13, 1982 to Kaun et al., assigned to the assignee of this invention, which is incorporated herein by reference. The electrode comprises a lithium-aluminum alloy and a ternary alloy of lithium, aluminum, and iron or cobalt in an amount sufficient to allow a five percent overcharge capacity.
A secondary cell and a method of making an electrode for said secondary cell was disclosed in U.S. Pat. No. 4,386,019 issued May 31, 1983 to Kaun et al., assigned to the assignee of this invention, which is incorporated herein by reference. Said electrode comprising a lithiumaluminum alloy, an electrolyte and a material selected from graphitized carbon, aluminum-iron alloy or mixtures thereof.
The disadvantage of previous cells incorporating a lithium-aluminum electrode was discussed in my previous patent, U.S. Pat. No. 4,446,212 issued May 1, 1984 assigned to the assignee of this invention, which is incorporated herein by reference. The invention therein includes the addition of an aluminum-iron alloy, and/or graphitized carbon and/or magnesium oxide to the negative electrode.
This invention provides a new Li-alloy electrode with improved cell performance and battery durability. The Li-SiNi electrode provides higher lithium activity at near full charge state for a lithium shuttle, creating overcharge tolerance with the added feature of having a higher cell voltage on discharge.